I'm no expert on these critters but here's what our Franklin is/was doing.
My two boys and I bought a red eared slider from the local pet store about 4 years ago and now it's grown to about 4 times the original size, I'd say it's shell is now about 12" front to back. Like yours when OK, ours seems to be normal and healthy, quite aggressive and eats anything it sees. We feed it mostly packaged dried turtle food, green Tetra repto-sticks supplemented with some red cichlid sticks I had left over but it seems to especially love those too, plus an earthworm now and then and any other bugs the boys catch as long as they clear it with me first. It's getting tight and I'll be moving it to a 55gal tank soon, but this guy or gal I'm not sure which still lives in just a 20gal tank with 6" of water and a single layer bed of 1/4"dia pebbles along the bottom and a pile of rocks on one end with a thick slab of tree bark over the rocks on which it suns/heats himherself under the 60w lightbulb reflector. I have pretty aggressive filtration to make sure the water stays very clean so I use a large Eheim canister filter.
Back during this past Winter, I got worried since it started to act lethargic, sleep all the time, didn't eat for a couple of months, just didn't look good and I thought it was gonna die

However, I had a suspicion that it might just be doing its hybernation thing or whatever turtles do in nature during the winter months. Happily, it must have been that kind of thing for Franklin since it fully "recovered" by late Spring and is now even larger and eats more and is back to its old self
Since I see that you are from "Somewhere in the background" and if I recall the Winters there are longer and end later than here in Pennsylvania, maybe your turtle's bio-clock is a little off and it'll recover and move on with life.
I hope this isn't too off topic but it is turtle related: last weekend I had my boys at Haverford School pond to catch frogs and feed the ducks, we were also netting small fish and throwing back whatever we caught, we just like catching stuff and looking at them. We also caught a couple of red eared slider "type" turtles, but I think up here in PA they are not the same as the one we have at home and are called something else, in my Field Guide of Reptiles, Snakes and Amphibians it says that red eared sliders are more a southern variety and do not normally find habitat above the median latitudes of the U.S. Suddenly, as we are at the shore of the pond, my younger boy yells "whoaaa what's that!" and I look to see a snapper turtle over 3ft in diameter with a huge head and beak it looked nasty enough to take your leg off! Both my boys said it looked like a dinosaur. We lured it partly out to get a good look but it wouldn't take any chances and kept going back under. I bet that thing snags ducks and geese off the pond!
I hope you find out about your turtle and all is well. I'll check back in case a knowledgeable turtle type here says something we should both know. Good luck.